During the past few years shaft sealing rings of elastomeric material often were replaced by shaft sealing rings whose sealing lips were made of polytetrafluoroethylene (PTFE) when intended for sophisticated applications, such as to be used in modem internal combustion engines. It has been suggested with such shaft sealing rings to provide a thread in a sealing portion thereof which will be in engagement with a surface area of the shaft, when the shaft sealing ring is mounted, so as to return the medium to be retained (lubricant) back into the space to be sealed when the shaft rotates (cf. H. K. Mueller “Abdichtung bewegter Maschinenteile”, 1990 Medienverlag Ursula Mueller, pages 42, 43, picture 18-RD).
A problem with such shaft sealing rings having a sealing lip of PTFE is that they seal only “dynamically”, in other words, when the shaft is rotating and, what is more, just in one direction of rotation. “Statically”, in other words with the shaft at standstill, they leak.
In connection with radial shaft sealing rings having an essentially radially oriented elastomer sealing lip, so-called hydrodynamic sealing aids are known which operate in the manner of “windshield wipers” (cf. pages 39, 40 and picture 14-RD of the book cited above by H. K. Mueller). Such shaft sealing rings, as a rule, are provided with a ring coil spring also known as “worm spring” to press the sealing lip against the periphery of the shaft. The radial contact pressure exerted by the worm spring, which pressure represents the contact pressure of the sealing lip in relation to the shaft periphery, normally lies in a range between 0.8 and 1.6 N/cm.
Faulty or forgotten mounting of the spring or its popping off while manipulating the shaft sealing ring before or during assembly of the shaft sealing ring is known to cause early failure. Elastomer sealing lip designs featuring a tip which engages the shaft in almost linear contact and under very high surface area pressure result in very high excess temperature and undesirable chemical reactions of the medium to be retained and/or the elastomer material of the seal and, therefore, cause damage in the contact zone of the sealing lip. That normally reduces the service life.
Also known is a radial shaft sealing ring, preferably made of PTFE which comprises a return flow means embodied by a groove adjacent the sealing edge. More precisely, this return means is composed of sinusoidal undulations oriented in circumferential direction and having a wedge-shaped inner profile, as seen in the direction of the space to be sealed (EP 0 798 498 B 1). This known radial shaft sealing ring is said to be effective regardless of the direction of rotation of the shaft to be sealed.
It is the object of the invention to provide a shaft sealing ring including a sealing lip of elastomeric material which will afford effective, reliable, dynamic sealing even at high circumferential speeds and vibrational loading of the shaft, irrespective of the direction of rotation thereof. It is another object of the invention to prolong the service life of the shaft sealing ring in comparison with known shaft sealing rings having an elastomer sealing lip and, furthermore, to warrant static sealing and to avoid premature failure. Moreover, a shaft sealing ring of the kind defined is to be easy to manufacture, thus allowing inexpensive production.
These objects are met by the shaft sealing ring disclosed herein.
A shaft sealing ring according to the invention has at least one undulating, closed, continuous return channel extending around the circumference of the sealing lip at least for a predetermined axial length throughout which the sealing lip engages the surface of the shaft. The return channel conveys exiting medium back to the space to be sealed while the shaft rotates.
Moreover, it assures that the shaft is wetted by the medium in the area of contact with the sealing lip, in other words it lubricates the shaft.
The flaccidly bendable or flexible design of the elastomer sealing lip allows a sealing portion of the sealing lip, by virtue of its flexural elasticity, to come to lie snugly tangentially against the periphery of the shaft across the predetermined length thereof, when mounted. The flexural elasticity is to be selected such that it will assure dynamic sealing in both directions of rotation of the shaft, without any need for the customary worm spring because the sealing portion will accompany vibrational motions of the shaft for being soft and pliable or flaccid.
The contact pressure is reduced, in comparison with a conventional solution according to which the sealing lip is pressed against the shaft surface at high specific pressure by means of a worm spring which acts merely through a tip. Here, the sealing portion engages the shaft surface in a surface area of predetermined axial length which covers the front edge at the side facing the medium and the rear edge at the side facing the surroundings of the first and other return channels, if any. In this manner friction is reduced so that excess temperatures which would damage the sealing lip cannot be generated any more. That contributes substantially to prolonging the service life.
The edge of the sealing lip facing the medium either may not be undulated, or it may be undulated in parallel with the one or more return channels.
A manufacturing advantage is obtained with both alternatives if the edge of the sealing lip facing the medium is provided, radially outside, with an axial cylindrical annular projection.
In that way the design of the mold for making the sealing lip is greatly simplified.
The “sealing edge” defined by the first return channel, including both its front and rear edges, is fully covered by the length “L” throughout which the sealing lip engages the shaft. Thus the medium which exits from the space to be sealed is reliably returned into that space. It is an essential feature of the invention that the rear edge facing the surroundings is pressed into contact with the periphery of the shaft by at least the same amount of pressure as the front edge of the first return channel which is oriented towards the medium. That is achieved by the selection of an angle of inclination “β” of the tangent to the front edge of the first return channel facing the medium and to the rear edge of this first return channel facing the surroundings or, in the case of a plurality of return channels, the last one upstream of the surroundings end. The sealing lip is to be dimensioned in such a way that this condition will be fulfilled even if the shaft sealing ring is installed eccentrically and/or the shaft rotates eccentrically.
According to the invention, therefore, the design engineer can adjust the contact pressure distribution as desired in the manner specified.
With sealing sleeves made of an elastomeric material having a hardness from approximately 70 to 80 IRHD, the angle β lies in a range between 0° and 5°, preferably between 1 and 3°.
The first return channel preferably is followed by other return channels, axially spaced from the first one in the direction towards the surroundings. In that event the tangent mentioned above touches the front edge of the first return channel facing the medium and the rear edge of the last return channel facing the surroundings.
The courses of the one or more return channels preferably are sinusoidal or composed of interconnected circular arc segments which are alternatingly curved in opposite directions.
A shaft sealing ring according to the invention effectively seals also when the shaft is not moving, in other words it seals statically. When the shaft rotates the shaft sealing ring seals dynamically in both directions of rotation of the shaft.
The fact that an additional contact pressure means, such as a worm spring is dispensed with not only reduces the contact pressure at which the sealing sleeve is pressed against the shaft, it also permits manufacture in one piece, whereby reliability is improved and production costs are lowered. Values applicable in practice for selecting the mean radial contact pressure referred to the periphery of the shaft for elastomer sealing sleeves are about 0.8 N/cm, so far the lowermost value obtainable for conventional radial shaft sealing rings. Preferably, they lie in a range between 0.1 and 0.4 N/cm.
Shaft sealing rings according to the invention are applicable, above all, in cases where sealing must be obtained in both directions of rotation and high excess temperatures must be avoided in the zone of contact between seal and shaft in order to prevent undesirable chemical reactions of the sealing ring with the medium to be retained. Such applications are given in motor vehicles, above all in gear shift mechanisms, gear drives connected downstream of converters, differential gears, and axles.
Advantageous further developments, in particular advantageous dimensioning measures are covered by the dependent claims.
The invention will be described further, by way of example, with reference to the accompanying drawings, in which: